The present invention pertains to a pneumatic automobile tire (1) which contains beads (2) with bead cores (3) arranged therein, wherein at least one sensor (S) is arranged in the pneumatic automobile tire (1). This sensor delivers signals that are correlated to the forces transmitted by the tire during its operation. Such information on tire forces serves for controlling the brakes and the chassis.
It is known to arrange sensors in tires, e.g. from DE 39 37 966 A1, DE 43 35 938 A1 and EP 06 02 679 B1. In these instances, the sensors are inserted between the base of the tread groove and the top belt layer.
All known sensors arranged in tires have the disadvantage of requiring an electric supply voltage. The supply voltage may be transmitted from the non-rotating car battery/on-board network to the rotating wheel via brushes, generated by means of a generator arranged in the rotating wheel (e.g., according to DE-OS 34 07 254) or stored in a battery or the like which is arranged in the rotating wheel.
According to the current state of the art, the arrangement of a round cell battery in the tire or the wheel rim appears the most advantageous option for providing the energy supply. However, most drivers only consider this feasible if the battery has a service life of more than three years. The most important factors for ensuring such a long battery service life are
a low power consumption per transmission signal (i.e., a very short transmission period per signal or very low transmission power) which, however, contradicts another objective, namely an absolutely reliable signal identification in the receiver, due to the multitude of weak electromagnetic waves being dispersed,
a low frequency of transmission signals per time unit (which appears feasible for monitoring the air pressure, but can only be utilized for traction control purposes by limiting the control precision),
a high battery capacity (which increases the weight and cost of the system),
a very low self-discharge current, and
a low internal resistance and a low voltage drop at decreasing temperatures down to 40xc2x0 C.
Although devices of this type were successfully tested in experimental automobiles, they have not been incorporated into a series production so far, wherein the incorporation into systems that also serve for traction control purposes, i.e., systems according to the present invention, currently appears improbable due to limitations of currently available batteries.
In addition, batteries of this type are difficult to exchange or can be easily stolen.
In order to solve this problem, EP-PA 03 63 570 already proposed to arrange the sensors that sense the longitudinal and/or lateral forces in a non-rotating system situated in the vicinity of the tire, e.g., on a transversal swinging arm of the wheel suspension, rather than in the tire. However, these measuring points are positioned relatively far from the tire contact areaxe2x80x94at which the transmission of forces between the automobile and the road takes placexe2x80x94namely even farther than in a conventional ABS system.
The elasticities, masses and oscillatory pulses between the sensors and the tire contact area (caused by stiffness fluctuations of rolling bearings and driving elements) diminish the precision of the measurement. The lower the precision, the farther the nominal slip between the tire tread and the road which is predetermined for the ABS or ESP system needs to lie below the critical slipxe2x80x94for maximum brake maneuversxe2x80x94in order to prevent overmodulation of deviation. This means that a correspondingly longer stopping distance must be accepted.
The principle of a non-rotating sensor arrangement is also utilized in DE-OS 44 35 160 A1 by the same applicant and the corresponding PCT/EP95/03864. However, the variable to be sensed, namely the side wall torsion, occurs in the rotating wheel. This principle makes it possible to determine deformations of the rotating wheel by means of non-rotating sensors arranged outside of the wheel. In this case, the forces are not directly measured, but rather the changes in time intervals, in which markings arranged on the tire pass by non-rotating sensors.
In comparison to ABS systems which were supplied with information via a magnet wheel until now, this system represents huge progress, namely because the mass of the wheel rim and the hub with its sleeve is no longer incorporated into the inertia calculation, i.e., this system allows faster control. However, applicants desired to broaden their knowledge and increase production by means of an internal development which resulted in a solution, in which the sensor is arranged in the rotating wheel.
The aforementioned measurement of the side wall torsion showed that torsional vibrations between the radially inner and the radially outer measuring track occur when continuously driving straight. Consequently, instantaneous torsion values showed intense fluctuations from the quotient longitudinal force/torsional rigidity. This influence can only be calculated by means of high resolution, i.e., a high pole number per magnetic track.
However, this effect also provides certain advantages, namely for detecting tread depth and aquaplaning, e.g., as described in the respective applications DE-OS 197 16 586.9 and 19 725 775.5. However, this effect also complicates the measurement of the longitudinal force which is of the utmost importance.
U.S. Pat. Nos. 4,625,207, 4,625,208 and 4,725,841 pertain to systems for obtaining signals from a passive transponder that carries phase-coded information.
EP 0 505 906 B1 discloses a pneumatic tire with an IC transponder that is arranged in the structure of the tire and used for tire identification, wherein an air pressure detector is incorporated into the transponder, and wherein said transponder is arranged on the axially inner side of the sealing inner liner together with its pressure detector.
DE-OS 41 12 738 discusses the fact that certain tire specifics vary from type to type. For example, tires of one manufacturer show a slightly different curve of the coefficient of friction xcexc as a function of slip than tires by another manufacturer, namely even if the tires have the same dimensions. The critical slip of one tire type may, in particular, be higher than that of another tire type.
Manufacturers of ABS systems usually attempt to achieve an optimal slip that is as high as possible, but sufficiently low for preventing the dreaded overmodulation of the brakes with all tires. This measure serves for preventing that, after a slightly excessive brake pressure, the brake pressure is excessively decreased, whereafter excessive brake pressure is built up again, etc. This means that the deviation is increased.
In such automobiles, this results in the tires which have the steepest xcexc-slip curve always performing best in brake tests, namely even if other tires reach a higher xcexc; the higher slip required by these other tires is, however, not even reached due to the cautious nature of the controller which is based on the steepest xcexc-slip curve stored in said controller.
The aforementioned DE 41 12 738 discloses a method for controlling and/or regulating automobile systems, wherein information on the properties of the actually mounted tiresxe2x80x94e.g., the xcexc-slip curvexe2x80x94is not stored in the controller, but rather in the tire so as to attain a superior tire identification system. In addition, this information is fed to the controller directly from the tire such that the controller bases the respective manipulations on the xcexc-slip curve that fits the individual tirexe2x80x94namely even if different tires are used.
According to one embodiment of this invention, a data carrier is arranged on the axially inner side of the inner tire bead.
The progress reports of the VDI (Association of German Engineers), Volume 8, No. 515, contains a report on the colloquium xe2x80x9cContactless Transmission of Measurement Data and Powerxe2x80x9d of Nov. 30, 1995 which was held at the College of Technology in Darmstadt and organized by Special Research Branch 241 of the German Research Association.
On pages 62-79 of this publication, L. Reindl and V. Magori of Siemens AG report on xe2x80x9cRadio Sensors with Passive Surface Wave Componentsxe2x80x9d and propose a signal-generating element which is referred to as a xe2x80x9csensorxe2x80x9d (but actually also contains other elements) and contains a rod, the length of which can be changed, wherein said rod is provided with a layer consisting of one or more piezoelectric crystals, e.g., silicon dioxide.
This element operates in accordance with the principle of passive radio transmission, i.e., without any other energy supply. In this case, an electromagnetic wave received from a transmitter is converted into an acoustic surface wave of the piezoelectric crystal layer, to which an electric wave of identical propagation speed is coupled due to the piezo property.
In this case, it is essential that the thusly determined propagation speed lies below the propagation speed of electromagnetic waves by approximately 5 powers of ten. Consequently, the reflection is delayed in such a way that it is not lost in the primary echo that follows the transmission pulse.
In order to realize the differentiation from the primary echo of the transmission signal, the term xe2x80x9cslow echoxe2x80x9d was used in this application; this refers to the echo, the energy of which is converted into a solid-borne sound wave in the preferred passive sensor before it is forwardedxe2x80x94in the form of an electromagnetic wavexe2x80x94to the receiver (that may be identical to the transmitter).
The delay depends on the length change and consequently the change in the distance traveled by the wave. However, the length change is proportionally linked to the bending stress and consequently the introduced force by Hook""s Law.
An article by Dipl.-Ing. Dr. Techn. A. Pohl, Dipl.-Phys. L. Reindl and Dipl.-Ing. H Scherr entitled xe2x80x9cWireless Measurements with Passive OFW Sensors on the Example of Monitoring Tire Air Pressurexe2x80x9d appeared on pages 305-317 of VDI Reports, collection No. 1350 which pertains to presentations at the 6th symposium xe2x80x9cTires, Chassis, Roadxe2x80x9d of the VDI Association xe2x80x9cAutomobile and Traffic Engineeringxe2x80x9d which was held on Oct. 23 and 24, 1997 in Hannover. This collection of reports was published by the VDI Publishing House, Dxc3xcsseldorf.
We believe that this is the first and thus far only publication that pertains to the utilization of an OFW sensor on a rotating wheel. In FIG. 5 on page 313 of this publication, an integrated OFW pressure measuring device is shown. According to the text portion on page 315, this pressure measuring device was arranged in the cavern of the tire, wherein a foil fixed to the tire flank was used as the sensor antenna. In lines 2 and 3 of page 316 of this report, it was already proposed to arrange the sensor element in the rubber of the tire.
Due to the aforementioned circumstances and the current state of the artxe2x80x94in which, depending on one""s point of view, either PCT/EP95/03864 or the last-mentioned VDI report are considered as being most closely related to the present mentionxe2x80x94it would be desirable to place a sensor for measuring the forces transmitted by the tire in such a way that a direct measurement of the forces, namely at least the longitudinal forces, can be realized in a particularly simple and low-weight fashion, namely without impairing the tire properties, in particular, its weight and its rolling resistance.
Thus, it is an object of the invention to directly measure the forces transmitted by the tire in an improved way.
The above and other objects of the present invention can be achieved in a pneumatic automobile tire (1) with beads (2) and bead cores (3) arranged therein, wherein at least one sensor (S) that delivers signals which are correlated to the forces transmitted by the tire during its operation is arranged in the pneumatic automobile tire (1). At least one of the sensors (S) that deliver signals which are correlated to the forces transmitted by the tire during its operation is arranged within the region of a bead (2).
In that case, all sensors (S) arranged within the region of a bead (2) should preferably operate in accordance with the principle of passive radio transmission i.e., these sensors should be able to alter or phase-shift radio signals received from a non-rotating device (G) in a defined correlation to the variable (F1 and/or Fq) to be sensed and transmit the altered or phase-shifted signals to a receiver (E), namely without any other type of energy supply.
The required receiver should be non-rotatably arranged in the vicinity of each wheel to be monitored, preferably in a position in which the receiver is fixed to the hub or the swinging arm.
The sensor arrangement according to the invention within the bead region has the disadvantage that the mechanical deformations to be sensed at this location are significantly smaller than, for example, the mechanical deformations occurring between the belt edge region and the bead region in the system according to PCT/EP95/03864. However, this sensor arrangement also provides certain advantages:
A detailed analysis of torsional tire vibrations showed that these torsional vibrations do not have the same intensity within all tire regions. In such a phase, for example, in which the oscillating movement of the tire tread area around the rotational axis lets the tire tread be in a hastening-forward-position, the wheel rim must be in a hastening-after-position. After half a period, the opposite analogously applies: When the osciallating movement of the tire tread areas around the rotational axis lets the tire be in a hastening-after-position, the wheel rim must be in a hastening-forward-position. The smaller the amplitude of the rim is, the greater its inertial mass is. Because the inertial mass of the rim differs from that of the tire tread, its amplitude differs from that of the tire tread, namely in that reciprocal manner explained above.
This led the inventors to conclude that an area which is almost free of torsional oscillation must exist between the tire tread surface and the wheel rim. The precise radius of the circle, on which the torsional oscillation amplitude becomes zero, depends on the mass distribution between the tire on one hand and the wheel rim with the hub and, if so required, the brake disk on the other hand. The precise radius of this circle also depends on the mass and stiffness distribution within these components, but the radius of this circle always lies slightly above the outer radius of the bead cores.
The precise radius of this circle which significantly simplifies the metrology while serving as the measuring location can be calculated in surprisingly accurate fashion with the conventional FEM (finite element method).
With respect to the preferred combination of the features of the invention, according to which all sensors (S) arranged within the region of a bead (2) operate in accordance with the principle of passive radio transmission, the slight deformations which can be detected at this location and were initially perceived as a disadvantage actually provide a synergistic advantage because this sensor type cannot be subjected to excessive deformations.
If a sensor is stiffer than the rubber surrounding the sensor, the field lines of identical stress are concentrated in the sensor. If a sensor is less stiff than the rubber surrounding the sensor, the sensor repels field lines of identical stress. Consequently, a concentration of field lines occurs within the rubber, namely in the vicinity of the sensor. This means that both constellations cause irregularities in the stress field and the deformation fieldxe2x80x94although irregularities of different types. The irregularities in the stress field lead to lateral stress concentrations in the rubber layer surrounding the sensor. The irregularities in the deformation field lead to eccentricities. In order to prevent these disadvantages, it would be most favorable if the stiffness of the sensor would correspond to the stiffness of the surrounding rubber in all directions; however, this cannot be achieved with 100% accuracy, but the bead represents the most favorable location for arranging sensors that operate in accordance with the surface wave principle because the stiffest rubber mixtures are used at this location.
The preferred combination of the characteristics of the invention does not require a magnetic pole track on the tire in order to generate data for all of the traction control systemxe2x80x94e.g., the brake control (ABS), the control of the driving torque and the slip adjustment that differs with respect to the wheel positions in order to prevent skidding (ESP).
Consequently, it is not possible for the magnetic track to be removed by abrasion, e.g., when the tire contacts a curb stone. In addition, a weight increasexe2x80x94which is unavoidable when embedding magnetizable particlesxe2x80x94is prevented.
The sensor arrangement within the bead region also results in a particularly low distortion of the calibration curves when standing waves occur in the tire tread and side wall regionxe2x80x94which is typical in the high-speed range.
The signal transmission path
a) between sensor and receiver (when using a central automobile transmitter) or
b) between transmitter and sensor and between sensor and receiver (when using one individual sensor transmitter per wheel whichxe2x80x94just like the receiverxe2x80x94should be positioned as close to the wheel as possible)
can be maintained relatively small while preventing contact of the components with one another if the wheel is subjected to high stress and correspondingly deformed; the signal transmission path may lie at 4 mm for race carsxe2x80x94and for passenger cars if the possibility of snow chains does not have to be taken into consideration. With respect to passenger cars that use snow chains, the signal transmission path should lie at approximately 13 mm, wherein the signal transmission path for trucks is correspondingly longer. Due to the fact that the required signal transmission path is maintained so short, the risk of receiving external signals is relatively low.
The sensor (S) according to the invention is preferably configured similar to a tongue, wherein its heel (Ws) is fastened to a bead core (3), and wherein the sensor extends radially outward from the bead core. The fastening of the exceptionally stiff bead core produces a connection with a very stable reference system. The radially outward protruding tongue deforms in the circumferential direction proportional to the longitudinal force transmitted by the tire and in the axial direction proportional to the transmitted lateral force.
In order to ensure that the slow echo has a sufficient intensity and can be adequately influenced, the sensor (S1, Sq)xe2x80x94i.e., the tongue coated with piezo crystalsxe2x80x94is made flat. According to invention, this tongue should, in addition to its radial extent, essentially have a certain axial extent in order to sense longitudinal forces. According to another feature of the invention, this tongue should, in addition to its radial extent, essentially have a certain extent in the circumferential direction in order to sense lateral forces.
In order to measure the longitudinal force transmitted by the tire and/or the tire deflection, the pneumatic automobile tire (1) according to a further detailed aspect of the invention preferably contains at least two sensors (S1) on a track near the bead core in uniform phase distribution; with respect to a redundance and a constant calibration curve as a function of the position of the rotational angle of the monitored wheel, a larger number of sensors, in particular, 3, would be even more favorable. The longitudinal force transmitted by the tire is correlated to the sum of the signals from the sensors (S1). It is important that all sensors (S1) used for this purpose have the same sensitivity.
In still a further aspect of the invention, in order to measure the lateral force transmitted by the tire and/or the tire deflection, the pneumatic automobile tire (1) preferably contains at least three sensors (Sq) of identical sensitivity on a track near the bead core, namely in uniform phase distribution.
It goes without saying that both aforementioned sensor quantities refer to the bead region, in the vicinity of which the receiver is arranged, i.e., usually the inner bead. If the tire to be monitored can also be mounted in reverse fashion, the suggested quantity of sensors naturally should be arranged within the left and the right bead region.
Thus, the pneumatic automobile tire according to the invention preferably contains several sensors for sensing longitudinal forces as well as several sensors for sensing lateral forces. Consequently a complete control of the tire slip is possible, as required for skid and roll prevention systems (ESP), anti-lock brake systems (ABS) and traction control systems.
In the previously known concurring system according to PCT/EP95/03864 by the same assignee, certain complications arise due to the fact that the mechanical calibration curve, i.e., the longitudinal force transmitted by the tire as a function of the tire side wall torsion, depends on the air pressure. In a highly inflated condition, a steel-belted radial tirexe2x80x94to which the invention primarily pertainsxe2x80x94not only behaves stiffer than in a less inflated condition with respect to its radial deflection, but also with respect to its elastic torsion between the tire tread and the beads. In a highly inflated condition, a higher longitudinal force occurs between the passing of the marks assigned to one another on different radii at the same torsional angle and consequently the same interval.
Although this effect of the aforementioned invention may prove usefulxe2x80x94namely for monitoring the air pressure or the wheel loadxe2x80x94if information on the transmitted longitudinal force or the effective wheel load or the existing air pressure is obtained otherwise, this particular solution is not offered in the form of a system consisting of individual modular options, but only as a complete package. Some customers consider this an unfair pricing policy by the manufacturer.
The present invention also fulfills the secondary requirement of achieving a practically constant calibration curve, at least within the air pressure interval +20% to xe2x88x9230% referred to as the nominal air pressure. For this purpose, the invention provides for arranging the radial center (Ssm) of each sensitive surface (Ss) at the radial distance from the bead core (3), at which the cross section of the carcass (4) has a point of inflection.
This additional development of the invention is based on the proposition that the dependence of the calibration curve on the air pressure is essentially not caused by, for example, the change in expansion (nearly no variation of stretch), but rather a curvature change of the strength due to a torsional moment, the plane, within which a strength carrier from bead to bead can be illustrated, extends slightly lateral to the radial if viewed in the form of a side view. Within this plane, the distance between the bead to the belt edge increases on both sides of the tire; however, since the carcass arc length between the bead and the belt edge increases less than that of the aforementioned increase in distance due to the high tensile rigidity of the strength carrier, the largest portion of the increase in distance results from the reduction in bulge of the strength carrier curve within the side wall region, namely in the transverse plane that now extends lateral to the radial, i.e., from a straightening in the sense of a reduction in bulge of the strength carrier.
However, an increase in the air pressure also causes stretching (=reduction in bulge) of the strength carrier. The less bulgy the cross section of the carcass is when no longitudinal forces are transmitted by the tire, the less likely it is that the tire will be additionally stretched due to longitudinal forces; the deformations per deformation force consequently become smaller, i.e., the tire exhibits stiffness.
However, a reduction in bulge cannot occur at locations at which no bulge exists, i.e., in the point of inflection of the carcass. With respect to the lesser deformations occurring at this location, only the shear strength G of the rubber, the tensile strength of the strength carrier and the corresponding thickness as well as tread densities are important, i.e., variables that entirely or at least essentially do not depend on the air pressure.
In an additional aspect of the invention, with an invariant air pressure calibration curve, it is proposed to realize a combination with other measuring systems that measure air pressure or at least deliver variables which depend on air pressure and can be compared to the defined variables that do not depend on the air pressure.
A combination with the solution according to the VDI Reports, supra, on pages 305-317 of collection No. 1350 appears particularly practical. In this case, the air pressure is determined by means of a load cell and a surface wave data transmission, wherein this method for measuring the air pressure is combined with the feature of passive radio transmission. This combination has the advantage that full operational monitoring can be realized without an electric power supply in the rotating wheel or any other additional means. However, a gradual displacement of the air pressure calibration scale must be expected over very long operating times due to air diffusion in the load cell.
In addition, a combination with the solution according to PCT/EP95/03864 may be considered because the evaluation of the difference between the tire forces, preferably the longitudinal tire forces, calculated by means of the two different methods makes it possible to deduce the tire air pressure without an electric power supply on the rotating wheel. However, this requires a magnetic track on the tire; consequently, sufficiently constant calibration curves can be ensured, namely even over long operating times.
The embodiment operating with the method of passive radio transmission is preferably additionally developed with respect to the fact that all sensors contained in the wheel operate with passive radio transmission. This means that no additional sensors which operate on a different principle should be arranged within the wheel so as to not lose the advantage of eliminating a rotating voltage supply and prevent false signal transmissions. If other sensors are used for monitoring the driving conditions or the air pressure, they should be arranged outside the wheel, preferably in non-rotating fashion, and operate in accordance with a different signal transmission method or at least a clearly different signal transmission frequency.
Although it is possible to determine system variables on one and the same tire with sensors of different design, such compromise solutions result in more disadvantages than advantages. Consequently, it is proposed to provide all sensors contained in the tire in the form of passive, linear acoustic surface wave components.
These OFW components arexe2x80x94if arranged within the bead regionxe2x80x94unexpectedly resistant in reference to the brisk accelerations and shocks to which a tire is subjected; sensors that operate in accordance with this principle appear to remain functional up into the high-speed range if arranged at this location.
According to another aspect of the invention, all sensors (S) contained in the pneumatic automobile tire (1) preferably contain a layer (Ssp) with one or more piezoelectric crystals, wherein said layer couples an electric wave of identical propagation speed to an acoustic surface wave. With respect to the temperature independence, the mechanical stability under load, the availability of manufacturing technology and the price, SiO2 is particularly suitable as a piezoelectric crystal.
As generally known with this measuring principle, at least one of the sensors (S)xe2x80x94preferably all sensorsxe2x80x94contains an interdigital converter (I) in order to increase the signal yield.
The transmission and reception frequency preferably lies between 20 MHz and 2.5 GHz.
Since no data processing can be carried out in the rotating system due to the passivity of the sensors, i.e., different data components need to be simultaneously received in order to process data, all sensors of one tire operate with transmission frequencies which differ from one another at least to such an extent that the respectively received signals can be separated from one another by carrier frequency. In this case, no discrete phase positions of the rotating wheel, in which the data transfer should take place, need to be determined in order to identify the data source; this means that a quasi-continuous measurement is possible.
The term xe2x80x9cquasi-continuousxe2x80x9d used in this application refers to the fact that the intervals between successive transmission pulses are shorter than the response times of the actuators. It was determined that it is possible for these intervals to have a duration of {fraction (1/50000)} second. Even at a speed of 180 km/h, i.e., 50 m/sec, and a wheel circumference of approximately 2 m, i.e., a rotary frequency of 25 Hz, a conventional ABS system or a system according to the aforementioned PCT/EP95/03864 would require an unrealistically high number of 2000 marks (approximately 60 marks are usually used) on the circumference in order to achieve a comparable chronological resolution. In the system according to the invention, the chronological resolution also remains identically high at slow speeds, but this chronological resolution decreases proportionately to the speed in both concurring systems.
For example, when driving on snow, i.e., at speeds around 60 km/h rather than 180 km/h, the invention allows very fast measurement and consequently very high control precision. At such a speed, the interval between the individual measurement results is approximately {fraction (1/100)} of the intervals attained so far.
The intervals between the individual measurement pulses could possibly be additionally reduced; in this case, the lower limit is defined by the fact that the delay in the OFW element needs to be sufficiently long such that the primary transmission pulse including its primary echo has already faded before the slow echo is sent back, and by the fact that the next primary transmission pulse is only transmitted after the slow echo has faded.
Due to the high chronological resolution of the measurement results obtained in accordance with the invention, it is also possible to additionally process these measurement results by means of analog technology instead of digital technology. It may be possible to additionally smooth out the signals by means of a simple circuitxe2x80x94e.g., a small capacitor and a resistancexe2x80x94as it is generally known from the pulsation damping following rectifiers. Until now, analog signal processing systems can be more easily realized with high insensitivity to shocks and temperatures than digital circuits.
It is particularly practical if a) a signal that is proportional to the torque, b) a signal that is proportional to the deflection and c) a signal that is proportional to the lateral force are obtained independently of the position of the rotational angle of the wheel. In this case, all devices for measuring the rotational anglexe2x80x94e.g., a magnet wheel in conventional ABS systems or a track of magnetic markings according to PCT/EP95/03864xe2x80x94can be eliminated. This reduces manufacturing expenditures, precludes possible errors and prevents signals of low resolution.
In a first approximation, the following can be stated for the local side wall torsionsxe2x80x94namely by neglecting the tire flatteningxe2x80x94if it is assumed that the bond between the belt and the tire tread forms a first quasi-rigid ring and the bead core on the wheel rim forms a second quasi-rigid ring:                               s          1                =                              V            M                    +                      Z            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (              phi              )                                                                        s          2                =                              V            M                    +                      Z            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                              phi                +                                                      1                    n                                    ⁢                                      xe2x80x83                                    ⁢                  360                  ⁢                  xc2x0                                            )                                                                        s          3                =                              V            M                    +                      Z            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                              phi                +                                                      2                    n                                    ⁢                                      xe2x80x83                                    ⁢                  360                  ⁢                  xc2x0                                            )                                                      ⋯                                    s          n                =                              V            M                    +                      Z            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                              phi                +                                                                            n                      -                      1                                        n                                    ⁢                                      xe2x80x83                                    ⁢                  360                  ⁢                  xc2x0                                            )                                          
wherein VM represents the rotation of the first ring (tire tread+belt) relative to the second ring (wheel rim+bead core) which is proportional to the torque, Z represents the eccentricity between both rings in the Z direction (vertical axis), and phi represents the instantaneous rotational angle of the wheel. In this case, s1 represents the composite deformation on the sensor S1 which can be detected in the side view, s2 represents the analogous composite deformation on the sensor S2 which can be detected in the side view, etc.
The Becherer-Kleinhoff formulas mentioned below can be derived from this statement, wherein said formulas allow, namely without knowledge of the rotational angle phi of the wheel, to quasi-continuously
a) measure the applied torque with only two sensors that respond to deformations in the circumferential direction (preferably three sensors so as to be able to utilize the same sensors as for xe2x80x9cb)xe2x80x9d, and
b) (also) measure the tire deflection with only three sensors that respond to deformations in the circumferential direction.
When using n uniformly distributed sensors (n represents a natural number greater or equal to 2), the torque M is determined by the equation:   M  =            C      M        ⁢          xe2x80x83        ⁢                            s          1                +                  s          2                +                  …          ⁢                      xe2x80x83                    ⁢                      s            n                              n      
In this case, Cm represent a calibration factor that essentially describes thexe2x80x94air pressure-dependentxe2x80x94torsional rigidity of the tire.
When using n uniformly distributed sensors (n represents a natural number greater or equal to 3), the tire deflection Z is determined from:   Z  =            C      Z        ⁢          2        ⁢          xe2x80x83        ⁢                                                      s              1              2                        +                          s              2              2                        +            …            ⁢                          xe2x80x83                        +                          s              n              2                                n                -                              (                                                            s                  1                                +                                  s                  2                                +                                  …                  ⁢                                      xe2x80x83                                    ⁢                                      s                    n                                                              n                        )                    2                    
In this case, Cz represents a calibration factor that describes the uniformity of the side wall torsion over the radial extent of the side wall. The more uniform the distribution, the closer Cz lies near 1. If the torsional rigidity is higher in the region in which the sensor extends than in the remaining region, Cz is greater than 1; this probably represents the most common instance. However, if the torsional rigidity is, by way of exception, lower in the region in which the sensor extends than in the remaining region, Cz is lower than 1.
In addition, the tire should contain three additional sensors in order to realize the preferred measurement of the complete tire load. These additional sensors respond to lateral deformations q and consequently allow a measurement of the cornering force. In a first approximation, the lateral deflection q of the side wall can also be described with a similarly simple statement xe2x80x94namely by neglecting tire flattening and lateral bending softness of both ringsxe2x80x94i.e., under the ideal assumption that all deformations occur exclusively in the side walls:                               q          1                =                              A            m                    +                      K            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (              phi              )                                                                        q          2                =                              A            m                    +                      K            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                              phi                +                                                      1                    n                                    ⁢                                      xe2x80x83                                    ⁢                  360                  ⁢                  xc2x0                                            )                                                                        q          3                =                              A            m                    +                      K            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                              phi                +                                                      2                    n                                    ⁢                                      xe2x80x83                                    ⁢                  360                  ⁢                  xc2x0                                            )                                                      ⋯                                    q          n                =                              A            m                    +                      K            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          (                              phi                +                                                                            n                      -                      1                                        n                                    ⁢                                      xe2x80x83                                    ⁢                  360                  ⁢                  xc2x0                                            )                                          
wherein Am represents the axial displacement of the first ring (tire tread+belt) relative to the second ring (wheel rim+bead core) which is averaged over the tire circumference and proportional to the lateral force, and wherein K represents the amplitude of the axial displacement path between both rings which is not constant over the circumference and is identical to the tire tread radius times the tilting angle between both rings. The tilting angle refers to the difference between the camber angle of the wheel rim and the camber angle of the tire tread. In other respects, phi also represents the instantaneous rotational angle of the wheel in this case. The reference symbol q1 represents the composite deformation on the sensor Q1 which can be detected in a view in the driving direction, q2 represents the analogous composite deformation on the sensor Q2 which can be detected in a view in the driving direction, etc.
This statement results in the following equations:                               A          m                =                                            q              1                        +                          q              2                        +                          …              ⁢                              xe2x80x83                            ⁢                              q                n                                              n                                        K        =                              C            K                    ⁢                      2                    ⁢                      xe2x80x83                    ⁢                                                                                          q                    1                    2                                    +                                      q                    2                    2                                    +                  …                  +                                      q                    n                    2                                                  n                            -                                                (                                                                                    q                        1                                            +                                              q                        2                                            +                                              …                        ⁢                                                  xe2x80x83                                                ⁢                                                  q                          n                                                                                      n                                    )                                2                                                        
In this case, the lateral force Fq will adjust itself independently of the wheel load as Fq=CFaxc2x7Am.
However, the lateral force Fq will adjust itself in dependence on the wheel load as Fq=CFkxc2x7K.
The ratio between CFa and CFk appears to be a signal that can be evaluated as a function of the wheel load. Although this signal only occurs while driving through curves, it allows a sufficiently accurate determination of the wheel load while driving straightxe2x80x94since the load hardly changes while driving. Information on the air pressure can be obtained with this knowledge of the wheel load in connection with the signal Z that describes the deflectionxe2x80x94which essentially depends on the ratio between the wheel load and the air pressure of the tire.
The previous explanations indicate that the sensor arrangement in the bead of the rotating tire makes it possible to describe all forces transmitted by the tire including the deformation (which is correlated to the wheel load) with a total of only six sensors, namely in quasi-continuous fashion as a function of time. This can be achieved independently of the position of the rotational angle of the wheel, i.e., in a non-rotating reference system. The rotational angle (phase position) consequently does not have to be determined when using 1t these measuring algorithms.
However, practical experiments showed less favorable results. The inventors believe that these inferior results are caused by the fact that the aforementioned equations are no longer entirely correct when a sensor passes through a phase region, in which the tire tread is flattened in contrast to the initially assumed ideal circumstances. Leaving aside the option of simply accepting these slight errors, there exist different methods for minimizing or entirely eliminating these inaccuracies:
A) The first option consists of using a slightly higher number (preferably 4 or 5) of sensors of each type, wherein the data of the sensors that are currently subjected to very fast changes are eliminated, and wherein the calculations are only carried out by utilizing the three remaining sensors of the respective type under such circumstances (in this case, the elimination frequency divided by n as the waste product results in a signal that is proportional to the wheel speed).
B) The second option consists of utilizing such a high number of sensors of each type that a constant number of sensors an essentially always situated within the contact area, i.e., the errors as a function of the time become essentially constant.
According to the experiences gained so far, strategy A is preferred.
In any case, the invention delivers a very dense, i.e., high resolution, and very accurate data base that can hardly be corrupted as a function of the time. Such an exceptional data base allows and utilizes particularly fast control algorithms and actuators. Traction control systems according to the invention consequently are able to adjust maximum traction on demand and require practically no safety margin with respect to critical slip. Where it has most meaning (e.g., on ice), a slip of 100% mayxe2x80x94naturally by almost completely relinquishing the steerabilityxe2x80x94be achieved, wherein the aforementioned slip results in the longest delay on ice.
After discussing the data processing, the following description pertains to data identification (i.e., the identification of the respective data source):
It isxe2x80x94although preferredxe2x80x94not absolutely imperative to utilize different carrier frequencies. The data identification is obtained by separating the signals sent back to the receiver due to the fact that the sensors of identical transmission frequency differ from one another with respect to the arrangement of their reflecting structures. The present invention provides for an instance in which all sensors (S1) for sensing longitudinal forces utilize a common transmission frequency f1 and all sensors (Sq) for sensing lateral forces utilize a common transmission frequency f2. It is also conceivable that f2=f1 if all sensors of one wheel differ from one another with respect to the arrangement of their reflecting structures.
All longitudinal force sensors (S1) of one tire, according to the invention, have a first, identical pattern of reflecting structures, and all lateral force sensors (Sq) have a second, identical pattern of reflecting structures. In this case, all longitudinal force sensors (S1) of one tire need to operate with different transmission frequencies fa, fb, fc and all lateral force sensors (Sq) also need to operate with the same transmission frequencies fa, fb, fc.
In an additional development of the carrier frequency identification discussed herein, it is also possible to assign a different carrier frequency to each sensor of all tires mounted on an automobile. This embodiment which is advantageous for the manufacture of prototypes is, however, not preferred for a series production because a different tire would be required for each wheel position, i.e., it would be extremely difficult to obtain appropriate replacement tires.
A superior method for realizing the signal identification consists of maintaining a small signal transmission path in relation to the wheel gauge and the wheel base. This method allows a separation of signals in accordance with different wheel positions by evaluating the signal intensity at the respective antenna.
According to the OFW technology, at least one antenna needs to be arranged in the tire. One particularly preferred antenna for this purpose consists of a concentrically arranged narrow ring of metal foil thatxe2x80x94if it is sufficiently narrow in relation to the side wall height and favorably placed in the radial directionxe2x80x94may be rigidly vulcanized onto the tire or even vulcanized into the tire rubber in order to lower the risk of injuries during operation and mounting of the tire.
The antenna should consist of a soft material that adheres well to rubber, i.e., brass foil is considered more suitable than steel foil. The average radius of such a foil ring for forming an antenna should preferably lie near or correspond to the radius, on which the cross section of the carcass has a point of inflection. Due to this measure, flattening effects which otherwise could become critical for the durability of the connection between the rubber and the metal are maintained very low. In addition, a synergistic combination characterized by the fact that the radial center (Ssm) of each sensitive surface (Ss) is arranged at the radial distance from the bead bore core (3) at which the cross section of the carcass (4) has a point of inflection results in the required cable lengths becoming very short or even zero.
If a separate antenna is assigned to each OFW sensorxe2x80x94in order to attain the previously described advantagesxe2x80x94such a metal foil ring can be divided into corresponding sectors by means of interruptions, wherein one sector is assigned to each respective sensor.
The sensor-equipped tires according to the invention as well as their additional developments serve for creating a friction control system for an automobile. In such axe2x80x94previously mentionedxe2x80x94friction control system in which all sensors (S) used operate on the same transmission frequency andxe2x80x94for the purpose of signal separationxe2x80x94at least one non-rotating transmission antenna (Gs) and a non-rotating reception antenna (Es) are assigned to each wheel position, and in which either the transmission antennas or the reception antennasxe2x80x94preferably the transmission antennas as well as the reception antennasxe2x80x94have a directional characteristic, each transmission antenna is preferably only excited in certain rotational positions of the pneumatic tire. In this case, the rotational positions naturally should be chosen such that the different sensors of one wheel do not simultaneously deliver signals, but rather in chronologically offset fashion such that signal separation is additionally promoted.
According to a more detailed aspect of the invention, each sensor (S1) for measuring longitudinal forces and, each sensor (Sq) for measuring lateral forces are preferably activated in a position vertically above and/or underneath the rotational axis of the wheel. In this case, only one sensor (S1 or Sq, respectively; also commonly referred to as xe2x80x9cSxe2x80x9d) suffices; however, in order to increase the resolution and the redundance, three sensors are preferably used for measuring the longitudinal forces and for measuring the lateral forces.
The longitudinal forces transmitted by the tire are largely correlated to the torsions occurring vertically above or underneath the rotational axis of the wheel, wherein said correlation is largely independent of the tire deformation. In addition, these longitudinal forces are also correlated to the sum of the torsions of the tire horizontally in front and behind the rotational axis.
The tire deformation is correlated to the difference between the torsions in the rotating direction of the tire horizontally in front or behind the rotational axis of the wheel. The tire deformation also provides information on the ratio between the wheel load and the tire pressure.
The last-mentioned effect is used for additionally increasing the safety of the automobile, namely with the aid of an additional development of the invention. According to this aspect of the invention, the sensors (S1) which serve for measuring the longitudinal forces arexe2x80x94in order to limit the expenditurexe2x80x94also utilized for measuring the tire deformation, wherein said sensors are activated in a different position in order to measure the tire deformation, namely horizontally in front and/or behind the rotational axis of the wheel. Only one sensor (S1) would suffice for this purpose; however, a larger quantity of sensors is preferred, in particular, 3.
For particularly high quality requirements, a high chronological resolution proves advantageous. If the sensors are only read in certain rotational positionsxe2x80x94which simplifies the data identification as described abovexe2x80x94such a high chronological resolution is most easily achieved with a very large quantity of sensors. However, an excessively high price for sensors, in particular, OFW sensors, contradicts the realization of such an embodiment.
An identical chronological resolution can also be realized with a smaller quantity of sensors if the quantity of read-out points is increased. However, the data obtained in positions which are not exactly situated vertically above or underneath or horizontally in front or behind the rotational axis can only be evaluated after linking the data with suitable trigonometric functions. In addition, the risk of a double receptionxe2x80x94i.e., an erroneous identificationxe2x80x94becomes higher the closer the various transmission and reception devices are arranged relative to one another. Consequently, no more than eight uniformly distributed angular positions should be used for such a measurement.
However, the inventors have recognized that it would be advantageous to pursue the previously described train of thought; if all sensors used in one wheel differ with respect to their transmission frequency and/or their reflection pattern in each individual datum of all the data delivered by this sensor can be assigned to one individual sensor. The acquisition of measurement data does not have to correspond to any angular positions in this case. This means that a quasi-continuous delivery of data is possible, i.e., a chronological resolution that lies far above that of all known systems.
However, the data processing, in particular, the preferred automatic calibration of the friction control system, would become more difficult due to the necessity of acquiring the respective rotational angle, wherein the signal with the lowest chronological resolution also defines the chronological resolution of the entire data set; however, the positions of the rotational angles can only be measured with an accuracy of approximately +/xe2x88x9230 in conventional systems (this corresponds to 60 markings on the circumference). This obstacle is remedied with the additional development which provides that the friction control system be characterized by the fact that no measurement of the rotational angle (phi) of the wheel is carried out, wherein the data delivered by the sensors of one wheel in quasi-continuous fashion are set into such a relation to one another in a logic circuit (e.g., with an algorithm according to the Becherer-Kleinhoff equations for M, Z, Am, K and Fq discussed herein) that data pertaining to a non-rotating coordinate system are delivered at the output of the logic circuit. Logic circuits of this type which merely need to carry out additions, subtractions, multiplications, divisions, squarings and taking roots are inexpensive, fast and reliable.
In order to verify the functionality of an OFW measuring system according to the invention, it is practical to separate the sensor surface, i.e., the piezo-coated plate, within which the acoustic-electric or acoustic wave should travel, from the rubber by means of a housing. Due to this measure, this wave is prevented from attenuating to such a degree by the hysteresis-caused attenuation on the surrounding rubber that the energy for the electromagnetic back-transmission is no longer available.
However, initial tests indicated that the hysteresis losses can be maintained sufficiently low for eliminating the aforementioned housing beginning at a transmitter carrier frequency of 200 kHz. Thus, the utilization of the OFW sensors should be limited to locations that are free ofxe2x80x94highly attenuatingxe2x80x94butyl rubber, i.e., not on or in the inner liner that ensures air tightness.